SUMMARY Multisensory integration is a fundamental function of the brain whereby the information collected through different sensory modalities is combined to form a unified percept. Defects in multisensory integration can affect perception and are a hallmark of many mental illnesses including autism spectrum disorders. Despite its fundamental role in the healthy and diseased brain, it remains unclear how multisensory integration is implemented in the brain, at the level of neuronal networks. This gap in our knowledge stems largely from the fact that multisensory integration has been primarily studied in the primate brain, where it is difficult to understand how neuronal activity patterns emerge from a specific connectivity architecture. In this proposal, we are putting forward a plan to investigate the basic mechanisms of multisensory integration using the Drosophila mushroom body as a model system. The mushroom body has been primarily investigated as an olfactory brain center but recent studies, including our own preliminary data, suggest that it is also a site for multisensory integration. The central hypothesis tested in this proposal is that the mushroom body integrates sensory information through two different mechanisms: an additive mechanism, whereby individual mushroom body neurons receive input only from only one sensory system and an integrative mechanism whereby individual mushroom body neurons integrate input from multiple sensory systems. In our preliminary analyses, we have identified the neurons projecting from different sensory centers ? including visual, olfactory, gustatory, thermosensory and hygrosensory centers ? to the mushroom body. We are proposing to test our leading hypothesis by pursuing three specific aims. First, we will determine how individual mushroom body neurons are connected to different sensory systems using a neuronal tracing technique we have developed. Second, we will determine how the entire population of mushroom body neurons responds to multisensory stimuli using calcium imaging. Third, we will determine whether, when learning complex multisensory stimuli, Drosophila learns individual features of these stimuli. Altogether, these three aims will provide anatomical, functional and behavioral evidence supporting our hypothesis. Once completed, this proposal will have delineated the basic mechanisms of connectivity and function underlying multisensory integration in the mushroom body. Given that many fundamental design principles of sensory systems are conserved between invertebrates and vertebrates, it is likely that the mechanisms of connectivity and function underlying multisensory integration in Drosophila will too be conserved in the more complex mammalian brain. The overarching goal of our research program is to apply our findings to a broader context: we believe that by understanding better how the numerically simple Drosophila mushroom body integrates, represents and transforms multisensory information, we will gain insight into how these mechanisms are implemented in the human brain and how their dysfunction can lead to defects in perception.